1. Field of the Invention
The present invention relates to an LED drive circuit for driving an LED (light emitting diode), as well as to an LED illumination fixture, an LED illumination device, and an LED illumination system having an LED as a light source.
2. Description of the Related Art
LEDs have such characteristics as low current consumption and long service life, and LED applications are expanding not only to display devices but to illumination fixtures and the like. In LED illumination fixtures, a plurality of LED units is often used in order to obtain the desired lighting intensity.
Common illumination fixtures usually use a commercial power supply (AC 100 V to 240 V), and in cases in which an LED illumination fixture is used in place of an incandescent lamp or other common illumination fixture, it is preferred that the LED illumination fixture also be configured to use a commercial power supply (AC 100 V to 240 V), the same as a common illumination fixture.
In the case of applying dimmer control to an incandescent lamp, a phase control dimmer (commonly referred to as an incandescent light control) is used in which dimmer control can easily be applied to the supply of power to the incandescent lamp by a single volume element, by switching on a switching element (usually a thyristor element or triac element) at a certain phase angle of an alternating-current power supply voltage. However, when a phase control dimmer is connected to a low-wattage incandescent lamp, flickering or blinking is known to occur, and normal dimming is not possible.
The ability to connect an existing phase control dimmer for an incandescent lamp without modification is preferred in cases in which dimmer control is applied to an LED illumination fixture that uses an AC power supply. By simply using existing equipment for dimming and changing only the fixture from an incandescent lamp to an LED illumination fixture, a significant reduction in power consumption can be realized relative to using an incandescent lamp. There is also no modification of the dimming equipment to create dimming equipment dedicated for an LED illumination fixture, and compatibility can be maintained, which leads to reduced equipment cost.
FIG. 20 shows an example of a conventional LED illumination system whereby dimmer control can be applied to an LED illumination fixture that uses an alternating-current power supply.
The LED illumination system shown in FIG. 20 is provided with a phase control dimmer 2, an LED drive circuit having a diode bridge DB1 and a current limiting circuit 23, and an LED module 3. The phase control dimmer 2 is connected in series between an alternating-current power supply 1 and the LED drive circuit.
In the phase control dimmer 2, the resistance value of a semi-fixed resistor Rvar1 varies, whereby a triac Tri1 is switched on at a power supply phase angle that is dependent on the resistance value of the semi-fixed resistor Rvar1. The semi-fixed resistor Rvar1 usually has a rotary knob configuration or sliding configuration, and dimmer control can be applied to an illumination fixture by changing the rotation angle of the knob or changing the sliding position of the semi-fixed resistor Rvar1. The phase control dimmer 2 is provided with a noise suppression circuit composed of a capacitor C1 and an inductor L1, and noise fed back to the AC power supply line from the phase control dimmer 2 is reduced by the noise suppression circuit.
FIGS. 21A through 21D show the waveform of the output voltage V2 of the phase control dimmer 2 and the waveform of the output voltage VDB1 of the diode bridge DB1 that correspond to phase angles of 0°, 45°, 90°, and 135° at which the triac Tri1 is on. As the phase angle at which the triac Tri1 switches on increases, the average value of the output voltage VDB1 of the diode bridge DB1 decreases, and the illumination fixture (LED module 3 in FIG. 20) decreases in brightness.
FIGS. 22A through 22C show examples of the waveform of the voltage V3 across the LED module 3 in the LED illumination system shown in FIG. 20. FIG. 22A shows the waveform of the voltage V3 across the LED module 3 at a bright dimmer level, FIG. 22B shows the waveform of the voltage V3 across the LED module 3 at a dark dimmer level, and FIG. 22C shows the waveform of the voltage V3 across the LED module 3 at an intermediate dimmer level (a level between the bright dimmer level and the dark dimmer level).
In a case in which the bright dimmer level is set, after the triac Tri1 switches from off to on, and the LED module 3 is lit, when the voltage V3 across the LED module 3 drops below the forward voltage VF of the LED module 3, current no longer flows to the LED module 3, and the triac Tri1 switches off. The voltage V3 across the LED module 3 therefore sharply decreases (see FIG. 22A).
FIG. 23A shows simulation waveforms of the voltage/current of each component of the LED illumination system shown in FIG. 20 in a case in which the bright dimmer level is set. FIG. 23A shows the waveform of the output voltage V1 of the alternating-current power supply 1, the voltage V3 across the LED module 3, and the current I3 flowing to the LED module 3. In the simulation results shown in FIG. 23A, the knob or slider of the semi-fixed resistor Rvar1 is set to the position at which the light intensity of the LED module 3 is at maximum; i.e., the resistance value of the semi-fixed resistor Rvar1 is set to 0Ω. In FIG. 23A, switching on of the triac Tri1 and rising of the voltage V3 across the LED module 3 occur when the phase is 53°. The light intensity of the LED module 3 is proportional to the average current of the LED module 3, and can therefore be estimated from the average current of the LED module 3. The relationship between the average current of the LED module 3 and the resistance value of the semi-fixed resistor Rvar1 is as shown in FIG. 24. Assuming the light intensity of the LED module 3 to be 100% in a state in which the phase control dimmer 2 is not provided, the light intensity of the LED module 3 is 90.5% in the conditions under which the simulation results of FIG. 23A are obtained.
On the other hand, in the case in which the dark dimmer level is set, after the triac Tri1 switches from off to on, and the LED module 3 is lit, when the voltage V3 across the LED module 3 drops below the forward voltage VF of the LED module 3, current no longer flows to the LED module 3. However, because the phase shift capacitors C2 and C3 are provided in the phase control dimmer 2, current flows to the triac Tri1 from the capacitors C2 and C3, and the triac Tri1 does not switch off (see FIG. 22B).
FIG. 23B shows simulation waveforms of the voltage/current of each component of the LED illumination system shown in FIG. 20 in a case in which the dark dimmer level is set. FIG. 23B shows the waveform of the output voltage V1 of the alternating-current power supply 1, the voltage V3 across the LED module 3, and the current I3 flowing to the LED module 3. FIG. 23B also shows the results of a simulation in which the resistance value of the semi-fixed resistor Rvar1 is 150 kΩ. In FIG. 23B, switching on of the triac Tri1 and rising of the voltage V3 across the LED module 3 occur when the phase is 141°. The light intensity of the LED module 3 is 0.71% in the conditions in which the simulation results of FIG. 23B are obtained.
For example, when the capacitance of the capacitor C2 is 100 nF, the resistance value of the resistor R1 is 5.6 Ωn, and the initial value of the voltage across the capacitor C2 is 141 V, the current flowing from the capacitor C2 takes approximately 900 μs to drop below the hold current (5 mA in this case) of the triac Tri1. Specifically, the holding time of the triac Tri1 by the capacitor C2 is approximately 900 μs. A waveform such as the one shown in FIG. 22A occurs in a case in which current stops flowing to the LED module 3 after 900 μs has elapsed since the triac Tri1 switched on, and a waveform such as the one shown in FIG. 22B occurs in a case in which current stops flowing to the LED module 3 within 900 μs. As shown in FIG. 22C, a waveform in which the waveform shown in FIG. 22A and the waveform shown in FIG. 22B are mixed occurs in a case in which conditions are exactly between those of the two waveforms, i.e., current stops flowing to the LED module 3 900 μs after the triac Tri1 switches on. When the state shown in FIG. 22C occurs, the amount of charge in the capacitor C2 and fluctuation of the time constants of the resistor R1 and capacitor C2 cause the current flowing to the LED module 3 to be unstable, the light flickers, and flickering occurs during low-level dimming.
FIG. 23C shows simulation waveforms of the voltage/current of each component of the LED illumination system shown in FIG. 20 in a case in which the intermediate dimmer level is set. FIG. 23C shows the waveform of the output voltage V1 of the alternating-current power supply 1, the voltage V3 across the LED module 3, and the current I3 flowing to the LED module 3. FIG. 23C also shows the results of a simulation in which the resistance value of the semi-fixed resistor Rvar1 is 135 kΩ. In FIG. 23C, the timing at which the triac Tri1 switches from off to on and the voltage V3 across the LED module 3 rises alternates between a phase of 137° and a phase of 141°. The light intensity of the LED module 3 is 1.58% in the conditions in which the simulation results of FIG. 23C are obtained.
The problem of flickering during low-level dimming described above generally occurs when the light intensity of the LED module 3 is about 1 to 5%, but because there are various types of dimmers, the range of 1 to 5% is merely an approximation, and the problem of flickering during low-level dimming occurs as well at intensities other than 1 to 5%.
Factors such as those described below also cause flickering in the LED module 3 in some cases. When the triac Tri1 of the phase control dimmer 2 switches from off to on, the LED module 3 switches from off to on, and the impedance of the LED module 3 rapidly changes. Ringing thereby occurs at an edge portion in which the output voltage of the phase control dimmer 2 rapidly changes, and during the period in which this ringing occurs, a phenomenon sometimes occurs in which the current flowing to the triac Tri1 oscillates several times between positive and negative, and the triac Tri1 subsequently switches off, after which a trigger occurs, the current flowing to the triac Tri1 again oscillates several times between positive and negative, and the triac Tri1 subsequently switches off This process is repeated, and the LED module 3 flickers.
In a case in which the illumination fixture is an incandescent lamp, since the load is a tungsten or other filament, even when the triac Tri1 of the phase control dimmer 2 switches from off to on, there is minimal fluctuation in the impedance, a state of low impedance is maintained, there is no rapid change in the current flowing to the phase control dimmer 2, and stable dimming operation is possible until the output voltage of the alternating-current power supply 1 is near 0 V.